Introduction to CT Brain: The Basic Principles

DR. WAN NAJWA ZAINI WAN MOHAMED

RADIOLOGIST AND HEAD,

JPD, HQE II

30-31 JULY 2015 HQE II BASIC CT BRAIN INTERPRETATION

HISTORY

BASIC PRINCIPLES

INDICATIONS, PREPARATIONS

ARTIFACTS

CT BRAIN is an extremely useful diagnostic

tool used routinely in hospital care.

Because many disease processes are time

dependent and require immediate action, a

quality physician needs to be able to accurately

interpret and act upon certain CT findings

without specialist (e.g., radiologist) assistance.

It has been shown that even a brief educational

intervention can significantly improve the

physician’s ability to interpret cranial CT scans.

Founder of X-ray

on 8th November,

German physicist,

Wilhelm Conrad

Röntgen

First X-Ray taken

– wife’s hand

Founder of CT in 1972

Sir Godfrey NewboldHounsfield

British Engineer in EMI, LTD.

“The Beatles’ greatest legacy”

Sir Allan McLeod Cormack

Physics Professor in US

Worked independently to develop solutions to mathematical problems

Both shared the Nobel Price in 1979

Original scanners took approximately 6

minutes to perform a rotation (one slice) and 20

minutes to reconstruct.

Current generation CT scans can complete a

full brain imaging in seconds.

Despite many technological advances since

then, the principles remain the same.

generation configuration detector beam Min scan time

first Translate -rotate 1-2 Pencil thin 2.5min

second Translate -rotate 3-52 Narrow fan 10sec

ThirdRotate- rotate 256-1000 Wide fan 0.5sec

fourth Rotate- fixed 600-4800 Wide fan 1sec

fifth Electron beam 1284 Wide fan

electron beam

CT - Computed Tomography

Tomography – a 2-D radiographic image

representing a slice through a body part. All

anatomy not at the target level is blurred.

CT scan – provides a 3D display of the

intracranial anatomy built up from a vertical

series of transverse axial tomograms.

Think like looking into a loaf of bread by cutting it into

thin slices and then viewing the slices individually.

Each tomogram represents a horizontal slice

through the patient’s head.

CT Scan combines X-Rays and detectors

coupled with a computer to create cross

sectional image of any part of the body.

CT X-Ray beams moves around the patient in a circular path.

The transmitted X-rays are received and absorbed by arrays of detectors across the patient on the opposite side of the circle from the X-Ray source.

Images are then reconstructed from the X-ray absorption data using mathemathicalprocesses.

Gantry

1. X-Ray tube

2. Generators

3. Collimators & Filters

4. Detector arrays & DAS

Patient couch

Computer console

1. X-ray tube &

collimator

2. Detector assembly

3. Tube controller

4. High freq. generator

5. Onboard computer

6. Stationary computer

Set scan parameters – kVp, mA, scan time, etc

Set scan mode – Digital radiograph, axial or volume

Houses reconstructor

Review and archive images

Post-processing

Cross sectional layer of

the body is represented

as an image matrix.

Each square of the

image matrix is called

pixel(picture element)

and it represents tiny

block of tissue called

voxel (volume element)

The average linear

attenuation coefficient (µ), between tube and

detectors

Attenuation coefficient

reflects the degree to

which the X-ray intensity

is reduced by a material

Air - 1000

Fat -70

Pure water 0

CSF +8

White matter +30

Gray matter +45

Blood +70

Bone/calcification +1000

Window width (W) : range of CT numbers

displayed in shades of gray, ranging from black

to white.

CT numbers > window : white;

CT number < window : black

Window level (L) : describes the centre of the

scale.

Allows the reader to focus on certain tissues

within a set of parameters. Most CT imaging

include windows that are optimized for brain,

blood and bone.

BRAIN : W 155, L 40

STROKE : W 30, L 30

SUBDURAL : W 150, L 5

BONE : W 3000, L 570

X-RAYS ARE ABSORBED TO DIFFERENT DEGREES BY DIFFERENT TISSUES

Always describe CT findings as densities –isodense/ hypodense/ hyperdense.

Higher the density = whiter is the appearance.

Lower the density = darker the appearance.

Brain is the reference density.

Anything of the density as brain = isodense.

Higher density than brain = hyperdense ( skull

is the best example).

Anything darker (lower density) than brain =

hypodense (CSF and air are classical

examples).

Acute head trauma

Suspected acute intracranial hemorrhage

Vascular occlusive disease (acute and chronic) or vasculitis(including use of CT angiography and/or venography)

Aneurysm evaluation

Detection or evaluation of calcification

Immediate postoperative evaluation following surgical treatment of tumor, intracranial hemorrhage, or hemorrhagic lesionS

Treated or untreated vascular lesions

Mental status change

Increased intracranial pressure

Headache

Acute neurologic deficits

Suspected intracranial infection

Suspected hydrocephalus

Certain congenital skull and brain lesions (such as, but not limited to, craniosynostosis, macrocephaly, and microcephaly)

Evaluating psychiatric disorders

Brain herniation

Suspected mass or tumor

CT guidance and image integration for neurosurgical, neurointerventional, and other therapeutic procedures

Certain skull lesions (such as, but not limited to, fibrous dysplasia, Paget disease, histiocytosis, osteolyticlesions,and skeletal tumors)

When MR imaging is unavailable or contraindicated, or if the supervising physician determines CT to be appropriate

Diplopia

Cranial nerve dysfunction

Seizures

Syncope

Ataxia

Suspicion of neurodegenerative disease

Developmental delay

Neuroendocrine dysfunction

Drug toxicity

Congenital morphologic brain abnormalities

Abusive head trauma and postmortem forensic investigations

Brain death

Suspected shunt malfunctions or shunt revisions

Informed consent.

Remove all metallic accessories, eyeglasses,

jewelleries, dentures, hearing aid.

Enquire about pregnancy, diabetes, renal

dysfunction, food allergies, asthma, cardiac

and other medical illness.

Avooid food at least 4 hours prior to a contrast

study.

Adequate hydration pre and post contrasted

Sedation – children, uncoorperative patients.

Avoid breasfeeding 24 hours after contrasted

study.

Equipped to deal with anaphylactic reactions.

ALARA concept.

Digital projection

– AP, PA, Lat or Oblique projection

– Surview, Scanogram

Conventional CT

– Axial

– Start/stop

Volumetric CT

– Helical or spiral CT

– Continuous acquisition

A scannogram/ topogram of the head is done

using digital projection method.

For head scans, conventional axial or slice by

slice method is commonly used.

For CTA studies, volumetric or helical/spiral CT

is used.

Artifacts are distortions or errors in the image

that are unrelated to the object scanned.

Most common artifacts in CT are:

Motion artifacts

Streak artifacts

Beam hardening artifacts

Partial voluming artifacts

Ring artifacts

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